V. DETERMINING FACTORS IN
CELL GROWTH
203
nevertheless active substances. For example, a large amount of an
extract of immature corn grains, yielded a small amount of a pure
compound, which could be completely interpreted in terms of one
molecule of indoleacetic acid combined to one molecule of arabinose
(Steward and Shantz, 1959a). This, however, is not a final identification
because the mode of attachment is not known, and there may well be a
number of isomers, depending upon the chemical configuration of the
sugar (i.e. whether it contains a pyranose or a furanose ring, or is linked
by an α or β linkage). These difficult questions require the isolation of
much larger amounts of the active substances and the eventual confirmation of their structure by synthesis. Similarly, the fruit of Aesculus
has yielded several substances of low molecular weight, relatively soluble
in organic solvents and separable by chromatographic methods, which
may be recognized as distinct entities by their ultra-violet absorption
spectra. At least four of these substances have been obtained, and one
of them, in its spectral properties, resembled chlorogenic acid. However,
this tentative identification is in some doubt, for it seems that the cell
division activity per se may be unrelated to the spectral properties of
chlorogenic acid, although it is associated with a portion that yields
caffeic acid and quinic acid on hydrolysis and so the substance may be a
derivative of chlorogenic acid.
Thus, from each source, a variety of growth substances may be
obtained; and these will cause carrot cells to grow when they are
present in very low concentration in the bathing medium and if they
are also in the presence of such substances as sorbitol and the two
inositols. Incomplete as the chemistry of these extracts may be, the
morphological interpretation is clear, for they all represent fluids which
nourish immature embryos and stimulate in the embryo shoot much
growth by cell division.
4. Synergistic Interactions and the Induction of Growth
Some explanted tissues cannot be cultivated on a basal medium which
contains coconut milk. Some of these, however, will grow if the medium
contains coconut milk and is supplemented by one of many synthetic
growth-regulating substances. The first substance to be effective in this
way was 2,4-D (2,4-dichlorophenoxyacetic acid), which, at a concentration of approximately 6 parts per million, will interact with coconut
milk to make the parenchyma of the potato tuber grow as a tissue
culture (Steward and Caplin, 1952). By contrast, similar explants from
the same tubers grow virtually not at all in media supplemented with
either coconut milk or 2,4-D alone. This system illustrates an important
point.
If the complete cell division-inducing principles are represented by
CELL GROWTH
203
nevertheless active substances. For example, a large amount of an
extract of immature corn grains, yielded a small amount of a pure
compound, which could be completely interpreted in terms of one
molecule of indoleacetic acid combined to one molecule of arabinose
(Steward and Shantz, 1959a). This, however, is not a final identification
because the mode of attachment is not known, and there may well be a
number of isomers, depending upon the chemical configuration of the
sugar (i.e. whether it contains a pyranose or a furanose ring, or is linked
by an α or β linkage). These difficult questions require the isolation of
much larger amounts of the active substances and the eventual confirmation of their structure by synthesis. Similarly, the fruit of Aesculus
has yielded several substances of low molecular weight, relatively soluble
in organic solvents and separable by chromatographic methods, which
may be recognized as distinct entities by their ultra-violet absorption
spectra. At least four of these substances have been obtained, and one
of them, in its spectral properties, resembled chlorogenic acid. However,
this tentative identification is in some doubt, for it seems that the cell
division activity per se may be unrelated to the spectral properties of
chlorogenic acid, although it is associated with a portion that yields
caffeic acid and quinic acid on hydrolysis and so the substance may be a
derivative of chlorogenic acid.
Thus, from each source, a variety of growth substances may be
obtained; and these will cause carrot cells to grow when they are
present in very low concentration in the bathing medium and if they
are also in the presence of such substances as sorbitol and the two
inositols. Incomplete as the chemistry of these extracts may be, the
morphological interpretation is clear, for they all represent fluids which
nourish immature embryos and stimulate in the embryo shoot much
growth by cell division.
4. Synergistic Interactions and the Induction of Growth
Some explanted tissues cannot be cultivated on a basal medium which
contains coconut milk. Some of these, however, will grow if the medium
contains coconut milk and is supplemented by one of many synthetic
growth-regulating substances. The first substance to be effective in this
way was 2,4-D (2,4-dichlorophenoxyacetic acid), which, at a concentration of approximately 6 parts per million, will interact with coconut
milk to make the parenchyma of the potato tuber grow as a tissue
culture (Steward and Caplin, 1952). By contrast, similar explants from
the same tubers grow virtually not at all in media supplemented with
either coconut milk or 2,4-D alone. This system illustrates an important
point.
If the complete cell division-inducing principles are represented by